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The electromagnetic force and strong and weak forces require particles like photons and gluons. But in case of gravity there is no such particle found.

Every mass bearing object creates a gravitational field around it, and whenever another mass bearing object enters its field the gravitational force comes into operation.

If all other forces of nature have some particles associated with them why should gravity be an exception?

And if there is no such particle, what exactly is the gravitational field and how does it spread over an infinite distance and cause the gravitational force to operate?

Note: I am a high school student and have not studied quantum mechanics.

Normie
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  • Relevant: http://physics.stackexchange.com/q/61899/ – jinawee Jun 03 '14 at 11:45
  • I asked this and got a nice explanation that helped me grok Gravity as a space-time concept: http://physics.stackexchange.com/q/81220/ – HC_ Jun 03 '14 at 17:31
  • Just because we haven't discovered these particles, it does not mean their not there. They could even be in the form of photons. There are alternate theories of push gravity that involve particles like these. – Bill Alsept Dec 12 '16 at 03:46

6 Answers6

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You're quite right that the other fundamental forces of Nature possess mediator particles, e.g. the photon for the electromagnetic force. For gravity, a graviton particle has been postulated, and is included in the five standard string theories which are candidates for quantum gravity. From a quantum field theory perspective, the graviton arises as an excitation of the gravitational field. String theory, of course, postulates it arises in the spectrum of a closed string.

Mass certainly gives rise to a gravitational field, but many other quantities do as well, according to the field equations of general relativity. As you're a high school student, I'll present them as,

$$\underbrace{G_{\mu\nu}}_{\text{geometry}}\sim \underbrace{T_{\mu\nu}}_{\text{matter}}$$

Spacetime geometry, and hence the gravitational effects, are equated to the matter present in a system, which may include energy, pressure and other quantities other than mass.

From a general relativity standpoint, the gravitational field may be viewed, or interpreted, as the curvature of spacetime, which is a manifold, i.e. surface. If we take space to be infinitely large, then the gravitational field must extend indefinitely; otherwise where would we choose to truncate? Even from a Newtonian perspective, we see that given the equation,

$$F_g \sim \frac{1}{r^2}$$

gravity must extend infinitely, as we never reach the point $r=\infty$ where it is truly zero.

As you asked, if the graviton is postulated, what is the need for a field? Well, we know that particle number is not conserved; we can have virtual particle and anti-particle pair production, and as such the idea that a field propagates throughout space, and the particles are excitations of the field, is a more compatible viewpoint. In addition, the concept of a field arises because of locality. From empirical evidence we know gravitation and electromagnetism do not act instantaneously, at every point.

JamalS
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  • But if a particle namely gravitron is postulated, then what is the necessity of the field concept? – Normie Jun 03 '14 at 10:03
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    @Avik: See the updated answer. – JamalS Jun 03 '14 at 10:06
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    @Avik: I've really tamed the answer down as you're a high school student, but let me know if you want the technical details regardless. – JamalS Jun 03 '14 at 10:16
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    @JamalS thank you for your time :) i am not sure i will be able to understand the technical details though – Normie Jun 03 '14 at 10:20
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    @Avik Field concept (in quantum field theory - QFT) is necessary for all particles: electrons, photons, quarks, etc. Particles are described as quanta (something like "elementary ripples") in fields. In QFT you cannot have a particle without a field: fields are the more fundamental, underlying concept. So you cannot have a graviton without a gravity field. Graviton is hypothesized as an "elementary ripple" in the curved spacetime. – mpv Jun 03 '14 at 12:25
  • @JamalS Does graviton have zero rest mass like the photon? If so, then it percieves zero travel time like the photon and experiences redshift? – this Jun 03 '14 at 14:08
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    @self.: The graviton is a massless gauge boson, and gravitational waves can experience the Doppler effect. – JamalS Jun 03 '14 at 14:33
  • can you give my answer a look and see if it's right? As far as I know, the SM with SU(5) can contain gravity. So the string theory is a part of SU(5) or independent with it, like two model which can include gravitation field? Thank you. – Ooker Jun 03 '14 at 23:16
  • Wait a minute - if there's a graviton, does that utterly negate the idea that "gravity is actually an 'apparent force' ..." – Fattie Jun 06 '14 at 08:40
  • @JoeBlow: Gravity is a genuine fundamental force, albeit with certain unique characteristics, and it can't be quantized like the others. – JamalS Jun 06 '14 at 08:43
  • Far out. Thanks for that, Jamal! Do you reckon my thinking correct that "the 'apparent force' analogy is utterly useless" ??? (My long comments in the 'populist answer' above!) Cheers! – Fattie Jun 06 '14 at 08:45
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    Just TBC: "Gravity is a genuine fundamental force" JamalS, you're saying that your position as an actual working physicist is that gravity "is a force"- you personally do not believe gravity is an 'apparent force'; it is not your position that gravity is an 'apparent force' ... (or, indeed, is that a hot/debatable/unknown/philosophical issue??) – Fattie Jun 06 '14 at 08:47
  • @JoeBlow Jamal probably wouldn't want to say anything like that outright, whether he believes it or not :) Real scientists tend to be rather cautious about saying that something is or isn't exactly right. If there is a graviton particle, that would lend a lot of counter-evidence to the "gravity as apparent force" interpretation. Graviton is still hypothetical - we have no experiment so far that would distinguish gravity with gravitons from gravity without gravitons. If gravity is a quantum field, graviton exists. And note that ST still uses curved spacetime, with more than GR's 4 dimensions. – Luaan Jul 18 '14 at 09:15
  • How do we know gravity and electromagnetism don't act instantaneously at every point? – Stan Shunpike Jan 27 '15 at 21:13
  • @StanShunpike As an example, consider the derivation of the Larmor formula for the power of a moving charged particle, it relies on the fact that the electric field does not propagate with infinite speed, i.e. instantly. It's precisely the time-dependent part of the electric field which is responsible for the electromagnetic radiation. – JamalS Jan 27 '15 at 21:48
  • @JamalS Oh I see what you mean. I was misinterpreting what you meant by instantaneously. I thought you meant that the force was sporadic over time, like going on and off. – Stan Shunpike Jan 28 '15 at 00:02
  • @StanShunpike Ah, I think you interpreted it as 'spontaneous' then rather than 'instantaneous.' – JamalS Jan 28 '15 at 06:17
  • @JamalS interesting. i'm not familiar with that term. of the fundamental forces, are any of them "spontaneous"? I associate spontaneous with chemistry. What does the term mean in physics? – Stan Shunpike Jan 28 '15 at 07:29
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    @StanShunpike Actually, the appropriate word for what you describe, the going on and off, is stochastic, i.e. random, and no, the fundamental forces aren't. – JamalS Jan 28 '15 at 07:38
  • String theory doesn't postulate the existence of a graviton. – Antoine Jun 20 '17 at 23:09
  • @user40085 In the spectrum of the string, there is a massless spin 2 boson, which has a particular vertex operator. If you apply perturbation theory to general relativity you will be able to recover this vertex operator. Moreover there is an argument as to why a spin 2 gauge boson can only correspond to the graviton. Know your facts before claiming things so boldly. – JamalS Jun 20 '17 at 23:39
  • @JamalS I know these facts, and that is precisely the point : string theory does not postulate the existence of graviton. The graviton appears in the spectrum of any string theory no matter what you try to avoid it, and this was a problem in the early time of dual models when people were not interested in having a string theory of gravity. So I stick to my claim : the graviton is not an postulate of string theory but a consequence. – Antoine Jun 25 '17 at 09:20
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Since you don't fully understand the answer of JamalS, I'll try to explain it shorter and easier for you.

If all other forces of nature have some particles associated with them why should gravity be an exception?

No, it isn't an exception. Physicists believe that the particle for gravity (called graviton) does exist, it's just they haven't found it yet. Standard Model doesn't have gravity, but extended Standard Model may have. Thanks to string theory.

What exactly is the gravitational field and how does it spread over an infinite distance and cause the gravitational force to operate?

It is exactly the space and the time. How do space and time appear? Big Bang. How does gravity operate? A change of space and time give you a gravitation force. Like a change in position gives you velocity ($v=\Delta x$), a change in energy gives you work ($W=ΔKE$). A change is very important, it will give you another interesting entity. If you have studied differential, you now know how important it is: describing a change.

By referring a change of space and time, I don't mean like a car travels through cities from morning to afternoon. I mean the car reforms the shape of time and space itself.

From Wikipedia:

Matter changes the geometry of spacetime, this (curved) geometry being interpreted as gravity.

enter image description here

Ooker
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I don't think the other answers have clearly called out that we do not know. Yes, we do have the (rather wonderful) theory of general relativity (GR), which does an excellent job of explaining the effect of gravity.

It does this by relating the presence of mass (strictly "stress-energy") to the structure of space-time. It also states how that effect propagates through space and time. So from a classical perspective, space-time itself can be seen as a gravitational field.

What it does not say is how space-time is able to interact with mass.

We expect that a quantum field theory type process is involved, and a lot of work goes in to determining this. Relating GR and quantum theory is in fact the fundamental problem of theoretical physics.

One of the key problems in solving this is in fact the very success of GR - we lack experimental evidence of it failing and hence providing a lead on where to improve it via quantum effects.

Peter Mortensen
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Keith
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    "I don't think the other answers have clearly called out that we do not know." Best answer here! Many young people, students, enquiring minds, etc, reading this: may simply not realise, this is the case. Just as Keith says. – Fattie Jun 05 '14 at 11:18
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    In retrospect, my physics education was overly good at focusing on what we do know with extremely little time spent explaining what we do not. "What we do not know" is where the real work is to be done in creating new science. It was (and probably is) possible to graduate top of your class and know nothing of this, at least from the curriculum. – Keith Jun 05 '14 at 23:21
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Gravity has a classical description called general theory of relativity (GTR), and it adequately describes the "force" of gravity as a consequence of space-time geometry.

However, a curved space around the gravitational body is an approximate description to a more precise quantum theory of gravity that will eventually replace GTR as it can be applied to micro or macroscopic systems. GTR replaced the Newtonian paradigm, but the description of gravity as a "pseudo force" takes it a step back. No one will ever say that a sky diver died because his parachute didn't deploy in that curved space.

For that matter the view of someone falling from the sky is a straight path and again fails to explain even Newton's gravitational constant. Curves take longer to traverse than straight lines and light being gravitationally lensed proves this. I have a lot more to say about this topic, but I will keep it to conventional means.

Brandon Enright
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Dan Park
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    Actually, GR works in the microscopic world as well. The problem integrating it with the standard model of physics is that it doesn't use mediator particles and quantum fields. It's very much possible that gravity simply isn't "quantum"-based, and that the attempts at finding a quantum theory of gravity are simply trying to find "particles" where they're not. GR doesn't "explain" the gravitational constant, but it is used in the proportionality constant - basically, the ratio between Enstein tensor and the energy-momentum tensor. It does relate spacetime structure to speed of light and gravity – Luaan Jun 05 '14 at 10:04
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Gravity as a field theory shows that particles move because of the curvature of spacetime - the field here is spacetime itself.

Electromagnetism is a field theory and light are just waves in the EM field which is cocontiguous with space that bears it.

Both the above are classical descriptions.

QM, and then QFT showed that we should quantise fields. This is how field quanta are shown; then we have the photon as the field quanta of the EM field, and also the graviton as the field quanta of essentially spacetime.

Whereas the photon has experimental support - the photo-electric effect and theoretical - QED; the same can't be said for the graviton.

The graviton shows up in the particle spectrum for string theory which is one reason why that theory is pursued.

Mozibur Ullah
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My theory is that gravity is the result of the incomplete cancellation of the atom's electromagnetic forces, due to the fact that there is a spatial separation between the charges. If this is correct, then the graviton would have "similar" properties as a photon, but very weak ($ Eg = Ep \times 10 ^{-39} $ ), which is the reason it has not been found.

Guill
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    That's a nice hypothesis. However, it would also mean that you're postulating that all of the charged and neutral particles (including the "fundamental" electron or neutrino) actually exhibit spatial separation of charges. That can be argued for atoms and, say, neutrons, but what evidence is there that it would be true for, say, neutrinos? Residual EM force from the weak force? Two charged particles still attract each other gravitationally (which was confirmed by experiments with different charges and masses of particles). – Luaan Jul 18 '14 at 08:56
  • Luaan, are you saying that there is some experiment that was capable of detecting a force difference of 10^-39? – Guill Sep 28 '14 at 08:28
  • Well, would the residual force obey the inverse square law? It doesn't seem likely, since the others we've observed (weak and strong nuclear force, Van der Waals' forces...) don't seem to. Yet gravity does. As for the experiment, you're probably right that I've mixed it up with scenarios which are complex enough to still allow your hypothesis to hold true. But there's many things it would have to explain to get hold - gravitational lensing, for example. – Luaan Sep 29 '14 at 07:18
  • Luaan, lensing would definitely be applicable. Anything that is applicable to an EM wave, would also be explained by this hypothesis. The reason for this, is that IT IS a "residual" EM wave. Yes, it also follows the inverse square law. I am working on an explanation for/at the quantum level. – Guill Apr 27 '16 at 03:04